Patent GB-1466732-A (GB)
Title: Magnetic corerection device for a cathode ray tube
IPC: H01J29/54
Filed: 1974-04-05
Abstract: 1466732 Cathode-ray tubes PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 5 April 1974 [9 April 1973] 15131/74 Heading H1D A magnetic correction device for a multibeam c.r.t. comprises at least one non-magnetic support 47, securing means for securing the support to the c.r.t. neck and at least one pair of coaxial rings having magnetic poles distributed about their peripheries, the rings being mounted on the support and rotatable about their axes in opposite relative directions, one of the two rings 43 of a pair having an inner diameter > the outer diameter of the other ring 45, the rings having teeth on their inner and outer peripheries respectively engaging with at least one pinion 53 rotatable about spindle (55) (Fig. 3, not shown) secured to support 47. Equal numbers of teeth are preferred. Colour purity correction is discussed w.r.t. Fig. 2a (not shown), convergence errors w.r.t. Fig. 2b (not shown) using quadrupolar fields produced by 4-pole rings (e.g. ring (27)) and sextupolar fields by 6-pole rings (Fig. 2c, not shown). The above construction enables axial length reduction as required for low depth colour c.r.ts. Rim (48) (Fig. 3, not shown) of support 47 centres the ring pairs. Lugs (57) and (59) facilitate rotation. Fig. 5 includes three supports 47 and ring pairs, with intermediate ring 61 fitting the tube neck with a small clearance, the inner diameter increasing towards one end to facilitate clamping. Conical clamping ring 63 has gap 64 which closes on insertion into the flare of ring 61. Rubber strips 65 increase friction and axial pressure is applied by ring 67 with projections 69 engaging collar 71, inclined rim sections 73 engaging ring 63 sections 75. The colour purity rings may be locked against rotation about ring 61. So that a single type support 47 may be either rotatable or locked, gaps (79) and protrusions 81 are provided. In Fig. 5, the middle support only is locked, and is flanked by rotatable supports with four and six pole rings. Knurled areas 83 prevent upsetting of a corrected condition and centre the outer rings. Axial thrust is by ring 85 having portion 87 whose thickness varies in the peripheral direction (developed portion 87, Fig. 7, not shown) and which is located between projection 89 on ring 61 and the left support 47. Knurled surface 92 prevents undue rotation. Rings 43 and/or 45 may also have knurled areas engaging knurled areas on supports 47 for locking. A braking block could be applied to the pins of supports 47 and rings 43, the block and pins also being knurled. Extension (93), Fig. 8 (not shown), facilitates rotation when lugs 57 and 59 are in equiangular positions. A higher or lower number of supports than that in Fig. 5 may be used and the c.r.t. may be other than side by side 3-gun type. The correction device is of low magnetic permeability or synthetic material as far as possible to prevent interference by the stray field from deflection unit (9) (Fig. 1, not shown). This applies also to rings 43, 45 and if of synthetic material, magnets may be locally secured to them (e.g. by an adhesive). The c.r.t. may include three side by side guns, for example, and a shadow mask and vertically striped phosphor screen. Low depth c.r.ts. are mentioned.